The present invention relates to microdischarge devices and, in particular, to novel structures for light emitting devices. It has long been known that electrical discharges are efficient sources of light, and today gas discharge lamps (including fluorescent sources, and metal-halide, sodium, or mercury arc lamps) account for most of the world's light-generating capacity (several billion watts on a continuous basis). Most of these devices are, unfortunately, bulky and frequently have fragile quartz or glass envelopes and require expensive mounting fixtures. In addition to general lighting, discharges produce ultraviolet and visible light for other purposes, such as germicidal applications (disinfecting surfaces and tissue), cleaning electronic and optical surfaces in manufacturing, curing polymers and activating light-sensitive molecules for medical treatments and diagnostics.
Although discharge devices were apparently first demonstrated by A. D. White in 1959, only recently were microdischarge devices fabricated in silicon by techniques developed in the integrated-circuit industry. As shown in FIG. 1, and see, for example, U.S. Pat. No. 6,016,027, a conventional microdischarge device 100 fabricated in silicon has a cylindrical cavity 102 in the cathode 104 of the device 100. The semiconductor cathode 104 was affixed to a copper heat sink with conductive epoxy. The anode 106 for the microdischarge device 100 was typically a metal film such as Ni/Cr. A thin dielectric layer 108 deposited onto the silicon electrically insulates the cathode 104 from the anode 106. When the cavity 102 is filled with the desired gas and the appropriate voltage imposed between the cathode 104 and the anode 106, a discharge is ignited in the cavity 102.
Microdischarges have several distinct advantages over conventional discharges. Since the diameter of single cylindrical microdischarge devices, for example, is typically less than 400–500 μm, each device offers the spatial resolution that is desirable for a pixel in a display. Also, the small physical dimensions of microdischarges allows them to operate at pressures much higher than those accessible to conventional, macroscopic discharges. When the diameter of a cylindrical microdischarge device is, for example, on the order of 200–300 μm, the device will operate at pressures as high as atmospheric pressure and beyond. Furthermore, at these high pressures, the microdischarge produces a stable, uniform glow. In contrast, standard fluorescent lamps, for example, operate at pressures typically less than 1% of atmospheric pressure.
Despite their applications in several areas, including optoelectronics and sensors, microdischarge devices can have several drawbacks. For example, the lifetime of the devices is exceedingly short, operating for only a few tens of hours. Damage to the anode is quickly visible and is caused by sputtering. Extracting optical power from deep cylindrical cavities is also frequently inefficient. If the cylindrical cathode for a microdischarge is too deep, it will be difficult for photons produced below the surface of the cathode to escape. In addition, conventional microdischarge devices may require fabrication techniques such as mechanical drilling and ablation. The use of these techniques limits the minimum size of the cavity diameter, thereby limiting the resolution of the devices. Furthermore, scaling an array of the devices is difficult as devices at the perimeter of large arrays may ignite preferentially.